Solution table

A common way to solve a sudoku is to use a solution table. This solution table containts all the numbers a cell can have. When starting on a sudoku the solution table contains the numbers 1-9 in all cells.

Reducing

When solving the sudoku you can reducing the numbers in the solution table using different "logical" methods. Then you cross over a number from the solution table to mark the number as not possible in that cell. When you have only 1 number left in a cell this number must be the solution for the cell.